Greatly speedup has_mate_threat()

Instead of loop across all legal moves to find a mate
loop across possible check moves only.

This reduces more then 10 times the number of moves to
check for a possible mate.

Also rename generate_checks() in generate_non_capture_checks()
so to better clarify what the function does.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>